Fluid pressure imprint lithography

ABSTRACT

An improved method of imprint lithography involves using direct fluid pressure to press the mold into a substrate-supported film. Advantageously the mold and/or substrate are sufficiently flexible to provide wide area contact under the fluid pressure. Fluid pressing can be accomplished by sealing the mold against the film and disposing the resulting assembly in a pressurized chamber. It can also be accomplished by subjecting the mold to jets of pressurized fluid. The result of this fluid pressing is enhanced resolution and high uniformity over an enlarged area.

FIELD OF THE INVENTION

This invention relates to imprint lithography and, in particular, toimprint lithography wherein direct fluid pressure is used to press amold into a thin film. The process is particularly useful to providenanoimprint lithography of enhanced resolution and uniformity over anincreased area.

BACKGROUND OF THE INVENTION

Lithography is a key process in the fabrication of semiconductorintegrated circuits and many optical, magnetic and micromechanicaldevices. Lithography creates a pattern on a thin film carried on asubstrate so that, in subsequent process steps, the pattern can bereplicated in the substrate or in another material which is added ontothe substrate.

Conventional lithography typically involves applying a thin film ofresist to a substrate, exposing the resist to a desired pattern ofradiation, and developing the exposed film to produce a physicalpattern. In this approach, resolution is limited by the wavelength ofthe radiation, and the equipment becomes increasingly expensive as thefeature size becomes smaller.

Nanoimprint lithography, based on a fundamentally different principleoffers high resolution, high throughput, low cost and the potential oflarge area coverage. In nanoimprint lithography, a mold with nanoscalefeatures is pressed into a thin film, deforming the shape of the filmaccording to the features of the mold and forming a relief pattern inthe film. After the mold is removed, the thin film can be processed toremove the reduced thickness portions. This removal exposes theunderlying substrate for further processing. Details of nanoimprintlithography are described in applicant's U.S. Pat. No. 5,772,905 issuedJun. 30, 1998 and entitled “Nanoimprint Lithography”. The '905 patent isincorporated herein by reference.

The usual method of pressing the mold into the thin film involvespositioning the mold and the substrate on respective rigid plates of ahigh precision mechanical press. With such apparatus, the process cangenerate sub-25 nm features with a high degree of uniformity over areason the order of 12 in². Larger areas of uniformity would be highlyadvantageous to increase throughput and for many applications such asdisplays.

SUMMARY OF THE INVENTION

An improved method of imprint lithography involves using direct fluidpressure to press a mold into a substrate-supported film. Advantageouslythe mold and/or substrate are sufficiently flexible to provide wide areacontact under the fluid pressure. Fluid pressing can be accomplished bysealing the mold against the film and disposing the resulting assemblyin a pressurized chamber. It can also be accomplished by subjecting themold to jets of pressurized fluid. The result of this fluid pressing isenhanced resolution and high uniformity over an enlarged area.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

FIG. 1 is a schematic flow diagram of the steps in an improved method ofimprint lithography;

FIG. 2 illustrates a typical mold and a substrate bearing a moldablefilm for use in the improved method of FIG. 1;

FIG. 3 illustrates apparatus for practicing the improved method of FIG.1;

FIGS. 4A, 4B and 4C show the moldable layer and substrate at variousstages of the process of FIG. 1;

FIGS. 5A, 5B and 5C illustrate various further processing steps that canbe performed on the substrate;

FIGS. 6A-6E illustrate alternative sealing arrangements useful in themethod of FIG. 1; and

FIG. 7 shows alternative apparatus for practicing the method of FIG. 1.

It is to be understood that these drawing are for purposes ofillustrating the concepts of the invention and are not to scale.

DETAILED DESCRIPTION

The use of a high precision mechanical press to press a mold into a thinfilm presents tolerance problems in replicating small patterns overlarge areas. Presses move on guide shafts through apertures, and thespacings between the shafts and their respective apertures can be largecompared to the features to be replicated. Such spacings permitundesirable relative translational and rotational shifts between thesubstrate and the mold. Moreover, despite the most careful construction,the molds and the substrates used in lithography are not perfectlyplanar. When these molds and substrates are disposed on the rigid platesof a press, the deviations from planarity over large areas can result invariations in the molding pressure and depth of imprint. Accordingly, itis desirable to provide a method of imprint lithography which avoids thelimitations of mechanical presses.

In accordance with the invention, the problem of unwanted lateralmovements of mechanical presses is ameliorated by using direct fluidpressure to press together the mold and the moldable surface. Theinventive methods apply fluid pressure over a surface of the mold, thesubstrate supporting the moldable surface or both. Because the fluidpressure is isostatic, no significant unbalanced lateral forces areapplied. Direct fluid pressure also includes fluid pressure transmittedto the mold or substrate via a flexible membrane, as it does notinterfere with the transmission of isostatic pressure from the fluid.And streaming pressurized fluid from numerous openings in a pressurevessel can also apply nearly isostatic direct fluid pressure on the moldor substrate.

It is contemplated that the invention will have important applicationsin the molding of a pattern on a previously patterned substrate. Themold can be aligned with the previous pattern using conventionalalignment techniques, and imprinting by direct fluid pressure minimizesany relative lateral shifts with consequent improvement in the alignmentof the two patterns.

Referring to the drawings, FIG. 1 is a schematic flow diagram of animproved process for imprint lithography using direct fluid pressure. Aninitial step shown in Block A, is to provide a mold having a pluralityof protruding features and a substrate-supported thin film of moldablematerial. The protruding features are preferably micrometer scalefeatures and, more advantageously, nanoscale features. The method ishighly advantageously where the mold surface has at least two protrudingfeatures spaced apart by at least one lateral dimension less than 200nm. A moldable material is one which retains or can be hardened toretain the imprint of protruding features from a mold surface.

FIG. 2 illustrates a typical mold 10 with protruding features and asubstrate 20 bearing a moldable thin film 21 for use in the process ofFIG. 1. The mold comprises a body 11 and a molding layer 12 including aplurality of protruding features 13 having a desired shape. The moldbody 11 and the molding layer 12 are typically fused quartz, glass orceramic. The molding layer 12 can be patterned into features 13 ofnanoscale dimensions using electron beam lithography and etchingtechniques well known in the art. The thickness of layer 21 is typicallyin the range 0.1 nm-10 μm, and the extent of protruding features 13 istypically in the range 0.1 nm-10 μm.

The substrate typically comprises a semiconductor wafer such as asubstantially planar wafer of monocrystalline silicon. The substratecould also be plastic, glass or ceramic. The moldable thin film 21 canbe any polymer that can be made pliable to pressure and can retain apressure-imprinted deformation or pattern. It can be a thermoplasticpolymer, such as polycarbonate or polymethyl methacrylate (PMMA), whichtemporarily softens in response to heat. Alternatively it can be aliquid, such as a UV-curable silicone, which hardens in response toradiation or a liquid which cures with heat. It can also be a compositelayer of polymer and hardenable liquid. The thin film is typicallyapplied to the substrate by spraying or spinning. Advantageously thefilm polymer does not adhere to the mold surface. If necessary, the moldsurface can be coated with a release agent to prevent such adherence.

In high resolution applications, the mold and the substrate areadvantageously made of the same material in order to minimizemisalignment due to differential thermal expansion or contraction.

Preferably the mold body 11, the substrate 20 (or both) is flexible sothat, under the force of fluid pressure, the mold and the substrate willconform despite deviations from planarity. Silicon substrates ofthickness less than 2 mm exhibit such flexibility for typical imprintpressures.

The next step, shown in Block B, is to place the mold and the thin-filmtogether and to seal the interface of the mold with the thin film,forming a mold/film assembly. If the thin film already includes apreviously formed pattern, then the pattern of the mold should becarefully aligned with the previous pattern on the film in accordancewith techniques well known in the art. The objective of the sealing isto permit external fluid pressure to press the mold into the film. Thesealing can be effected in a variety of ways such as by providing a ringof material such as an elastomeric gasket around the area to be moldedand peripherally clamping the assembly.

The third step (Block C) is to press the mold into the film by directfluid pressure. One method for doing this is to dispose the assembly ina pressure vessel and introduce pressurized fluid into the vessel. Theadvantage of fluid pressure is that it is isostatic. The resulting forceuniformly pushes the mold into the thin film. Shear or rotationalcomponents are de minimus. Moreover since the mold and/or substrate areflexible rather than rigid, conformation between the mold and the filmis achieved regardless of unavoidable deviations from planarity. Theresult is an enhanced level of molding resolution, alignment anduniformity over an increased area of the film.

The pressurized fluid can be gas or liquid. Pressurized air isconvenient and typical pressures are in the range 1-1000 psi. The fluidcan be heated, if desired, to assist in heating the moldable thin film.Cooled fluid can be used to cool the film.

FIG. 3 illustrates a sealed mold/film assembly 30 disposed within apressure vessel 31. The assembly 30 is sealed by a peripheralelastomeric gasket 32, extending around the area to be molded. Theperiphery of the assembly can be lightly clamped by a clamp (not shown)to effectuate the seal. The vessel 31 preferably includes avalve-controlled inlet 34 for the introduction of pressurized fluid anda valve controlled outlet 35 for the exit of such fluid. The vessel 31may optionally include a heater 36 for heating a thermoplastic or heatcurable thin film and/or a transparent window 37 for introducingradiation to cure or cross link the film. A sealable door 38 can provideaccess to the interior of the vessel.

The next step shown in Block D, is to harden the moldable thin film, ifnecessary, so that it retains the imprint of the mold and to remove themold. The process for hardening depends on the material of the thinfilm. Some materials will maintain the imprint with no hardening.Thermoplastic materials can be hardened by preliminarily heating themprior to molding and permitting them to cool after imprint. PMMA, forexample, can be suitably softened by heating to 200° C. prior to moldingand hardened by cooling after imprint. Heat curable materials can behardened by applying heat during imprint. The above-described eater 36and/or the use of a heated pressurized fluid can effectuate suchhardening. Radiation curable materials can be hardened by theapplication of UV radiation during imprint. Such radiation can besupplied through the window 37 of the pressure vessel. The mold can bemade of transparent material to permit the radiation to reach the film.Alternatively, the substrate can be transparent and the windowpositioned to illuminate the film through the substrate.

The fifth step shown in Block E is optional in some applications. It isto remove contaminants (if any) and excess material from the recesses ofthe molded thin film. The molded film will have raised features andrecesses. In many lithographic operations it is desirable to eliminatethe material from the recesses so that the underlying substrate isexposed for further processing. This can be conveniently accomplishedusing reactive ion etching.

FIGS. 4A, 4B and 4C show the moldable layer and substrate at variousstages of the process. FIG. 4A illustrates the layer 21 duringimprinting by mold 10 pressed by fluid pressure in the direction ofarrow 40. The protruding features 13 of the mold press into layer 21,producing thinned regions 41. The recessed regions 42 of the moldbetween successive protruding features leave layer 21 with regions 43 ofgreater thickness.

FIG. 4B shows the layer 21 after hardening and removal of the mold. Thelayer 21 retains the thinned regions 41 and thick regions 43 inaccordance with the pattern imprinted by the mold.

FIG. 4C illustrates the layer and substrate after removal of the excesslayer material in the recesses, exposing nanoscale regions 44 of thesubstrate 20.

In important applications the resulting structure is a resist-coveredsemiconductor substrate with a pattern of recesses extending to thesubstrate as shown in FIG. 4C. Such a structure can be further processedin a variety of ways well-known in the art. For example, the molded filmcan be used as a mask for the removal of surface layers in exposedregions of the substrate, for doping exposed regions or for growing ordepositing materials on the exposed regions.

FIGS. 5A, 5B and 5C illustrate such further processing. In FIG. 5A, thesubstrate can include a surface dielectric layer 50 (such as SiO₂ on Si)and the mask layer can permit removal of the dielectric at exposedregions. In FIG. 5B impurity regions 51 can be diffused or implantedinto the semiconductor selectively in those regions which are exposed,altering the local electrical or optical properties of the dopedregions. Alternatively, as shown in FIG. 5C new material layers 52 suchas conductors or epitaxial layers can be deposited or grown on theexposed substrate within the recesses. After processing, the remainingmaterial of the molded layer can be removed, if desired, usingconventional techniques. PMMA, for example, can be cleaned away bywashing with acetone. A substrate can be subjected to additionallithographic steps to complete a complex device such as an integratedcircuit.

As mentioned above, there are a variety of ways of sealing the mold/filmassembly 30 so that pressurized fluid will press the mold into the film.FIGS. 6A-6D illustrate several of these ways.

FIG. 6A schematically illustrates an arrangement for sealing a mold/filmassembly by disposing the assembly within a sealed covering of flexible,fluid-impermable membrane 40 (e.g. a plastic bag). In this arrangementthe region between the mold and the moldable layer is sealed in relationto an external pressure vessel. Preferably the air is removed from thebag before molding.

FIG. 6B shows an alternate sealing arrangement wherein the assembly 30is sealed by a peripheral sealing clamp 61 which can be in the form of ahollow elastic torroid. Sealing can be assisted by providing the moldwith a protruding region 62 extending around the region to be molded. Inuse, the clamp and pressurized fluid will press the protruding region 62into the moldable film, sealing the molding region. FIG. 6C illustratesa sealing arrangement in which the assembly 30 is sealed by applying aperipheral tube or weight 63 which lightly presses the mold onto themoldable film. A peripheral protruding region 62 can assist sealing.

FIG. 6D shows an alternative sealing arrangement wherein the assembly 30is sealed by a sealing o-ring 64 between the mold and the substrate.Preferably the o-ring seats within peripheral recesses 65, 66 in themold and the substrate, respectively. Light pressure from a peripheraltube or weight 63 can assist sealing.

FIG. 6E shows yet another sealing arrangement in which the assembly 30is disposed between flexible membranes 40A and 40B is enclosed within apair of mating cylinders 67A, 67B. Application of fluid pressure to theinterior of the cylinders would press the mold and moldable surfacetogether.

Alternatively, two the cylinders could lightly seal against the mold andthe substrate, respectively, before pressurization. Yet further in thealternative, the substrate could rest upon a support and a singlecylinder lightly seal against the mold or a membrane.

FIG. 7 illustrates alternative molding apparatus 70 where the assemblyis disposed adjacent openings 71 in a hollow pressure cap 72 and themold 10 is pressed into the moldable layer 21 by jets of pressurizedfluid escaping through the openings 71. The cap 72 (analogous to vessel31) has an internal chamber 73 for pressurized fluid. The region betweenthe mold and the moldable film is effectively sealed from the pressurevessel by the upper surface of the mold.

In operation, the substrate and mold are placed on a substrate holder79. The cap 72 can be held in fixed position above the mold 10, as bybars 74, 75. High pressure fluid, preferably gas, is pumped into chamber73 through an inlet 76. The high pressure fluid within the chamberproduces a fluid jet from each opening 71. These jets uniformly pressthe mold 10 against the moldable layer to imprint the mold features.

Advantageously, the cap 72 can include a groove 77 along a perimeter ofthe face adjacent the mold 10. The groove 77 can hold an o-ring 78between the cap 72 and the mold 20. The o-ring decreases fluid outflowbetween the cap 72 and the mold 10, increasing the molding pressure andmaking it more uniform.

Alternatively, the substrate holder 79 can have the same structure ascap 72 so that the substrate is also pressed by jets of pressurizedfluid.

EXAMPLES

The invention may now be better understood by consideration of thefollowing specific examples.

Example 1

A silicon wafer of 4″ diameter is coated with a layer of PMMA 150 nmthick. A mold is made of a 4″ diameter silicon wafer and has pluralsilicon dioxide protruding patterns 100 nm thick on one side of itssurface. The mold is placed on the PMMA layer with the protrudingpatterns facing the PMMA. The mold and the substrate are sealed in aplastic bag within a chamber, and the chamber is evacuated. Thennitrogen gas at 500 psi is introduced in the chamber. A heater in thechamber heats the PMMA to 170° C., which is above the glass transitiontemperature of the PMMA, softening the PMMA. Under the gas pressure, themold is pressed into the PMMA. After turning off the heater andintroducing a cold nitrogen gas, the PMMA temperature drops below itsglass transition temperature, and the PMMA hardens. Then the nitrogengas is vented to the atmosphere pressure. The mold and substrateassembly is removed from the chamber. The bag is cut off, and the moldis separated from the substrate.

Example 2

A silicon wafer of 4″ diameter is coated with a layer of PMMA 150 nmthick and is placed on a chuck. The chuck has a plurality of small holeson its surface. The holes can be connected either to vacuum or topressurized gas. When the holes are connected to vacuum, the chuck holdsthe wafer on the chuck's surface. A mold made of a 4″ diameter siliconwafer has a plurality of silicon dioxide protruding patterns (100 nmthick) on one of its surfaces. The mold is held by a second chuck, whichhas the same design as the substrate chuck. The mold is placed on top ofthe PMMA layer with the protruding patterns facing the PMMA. The moldand the substrate are placed in a chamber. The PMMA can be heated fromthe chuck.

During the imprint process, the PMMA is first heated above its glasstransition temperature. A ring pattern on the mold seals off the moldpattern inside the ring from the pressure outside. Then the holes ofboth chucks are changed from vacuum to a gas pressure of 500 psi. Thepressurized gas presses the mold protruding patterns into PMMA.Importantly, the pressurized gas presses the mold protruding patterninto the PMMA uniformly in submicron scale, despite the roughness of thebacksides of the mold and the substrate as well as the roughness of thechuck surfaces.

It is to be understood that the above described embodiments areillustrative of only a few of the many embodiments which can representapplications of the invention. Numerous and varied other arrangementscan be made by those skilled in the art without departing from thespirit and scope of the invention.

What is claimed:
 1. A method for processing a surface of a substrate comprising the steps of: applying a moldable layer on the surface of the substrate; providing a mold with a molding surface having a plurality of protruding features; pressing the molding surface and the moldable layer together by direct fluid pressure to reduce the thickness of the moldable layer under the protruding features to produce reduced thickness regions; and withdrawing the mold from the moldable layer.
 2. The method of claim 1 further comprising the steps of: removing the material of the moldable layer from the reduced thickness regions to selectively expose regions of the substrate; and further processing the substrate selectively in the exposed regions.
 3. The method of claim 2 wherein the further processing comprises doping the substrate with impurities, removing material from the substrate, or adding material on the substrate.
 4. The method of claim 1 further comprising the step of hardening the moldable layer after pressing.
 5. The method of claim 1 wherein the pressing comprises sealing a region between the mold and the moldable layer and subjecting the mold and the substrate to pressurized fluid.
 6. The method of claim 5 wherein the sealing comprises sealing a region between the mold and the substrate from the pressurized fluid.
 7. The method of claim 1 wherein the substrate or the mold or both are sufficiently flexible to conform together under the fluid pressure.
 8. The method of claim 1 wherein the pressing comprises pressing the mold and moldable layer together by streaming pressurized fluid.
 9. The method of claim 1 wherein at least two of the protruding features of the molding surface are laterally spaced apart by less than 200 nm.
 10. The method of claim 1 where the thickness of the moldable layer is in the range 0.1 nm to 10 μm.
 11. A process for patterning a mask layer on a semiconductor substrate comprising: applying the mask layer to the semiconductor substrate; disposing a mold having a patterned surface adjacent the mask layer; filling a chamber with pressurized fluid; and subjecting the mold or the substrate to pressurized fluid from the chamber to press together the mold and the mask layer.
 12. The process of claim 11 wherein the material of the mask layer comprises a polymer and further comprising the step of curing the polymer after performing the pressing.
 13. The process of claim 12 wherein the curing includes illuminating the layer with radiation.
 14. The process of claim 12 wherein the cured mask layer conserves an imprinted pattern from the mold.
 15. The process of claim 11 further comprising cooling the mask layer to a temperature at which the material of the mask layer hardens.
 16. The process of claim 11 wherein the material of the mask layer comprises resist.
 17. The process of claim 16 wherein the material of the mask layer comprises a liquid polymer.
 18. The process of claim 11 further comprising heating the mask layer prior to pressing to a temperature at which the material of the mask layer is pliable.
 19. The process of claim 11 wherein the pressing comprises applying the fluid pressure to a surface of the mold to push the patterned face of the mold towards the substrate.
 20. The process of claim 11 the pressing comprises applying the fluid pressure to a surface of the substrate to push the substrate towards the patterned face of the mold.
 21. The process of claim 11 further comprising: removing the mold from the mask layer leaving molded recesses in the mask layer; and cleaning the mask material from the molded recesses to expose regions of the substrate.
 22. The process of claim 21 further comprising one or more of the following steps: a selective etch of the exposed substrate, a selective diffusion of impurities into the exposed substrate, and a selective deposition of material on the exposed substrate.
 23. The process of claim 11 further comprising positioning a sealing material to isolate a region between the mold and the mask layer from the fluid pressure in the chamber.
 24. The process of claim 23 wherein the positioning includes placing a ring of material around a region between the mold and the mask layer.
 25. The process of claim 23 wherein the positioning comprises placing at least one flexible membrane between the pressure chamber and at least one of the mold and the substrate.
 26. A process of treating a semiconductor substrate, comprising the steps of: disposing a layer of mask material on the substrate; positioning a mold with a patterned surface adjacent the layer of mask material; positioning a sealing device to isolate the layer of mask material from a pressure chamber; disposing the masked substrate and the mold in a pressure chamber; and increasing a pressure of pressurized fluid in the pressure chamber to force together the patterned face of a mold and the layer of mask material.
 27. The process of claim 26, wherein the positioning of the sealing device hermetically isolates a region between the layer of mask material and the mold from pressurized fluid in the pressure chamber.
 28. The process of claim 26 further comprising heating the mask layer prior to the increasing of pressure.
 29. The process of claim 26 further comprising curing the mask layer after the pressing to harden deformations caused by the mold.
 30. The process of claim 29 further comprising removing the mold from contact with the mask layer after the curing.
 31. The process of claim 30 further comprising the step of removing contaminants from the mask layer after removing the mold.
 32. The process of claim 29 further comprising cleaning the mask material from the deformations.
 33. The process of claim 32 including further processing the substrate by one or more of the following steps: selectively etching from the substrate, selectively doping impurities in the substrate, and selectively adding material on the substrate.
 34. The process of claim 26, wherein increasing the pressure comprises applying pressure to a fluid in the chamber.
 35. The process of claim 26 where the fluid comprises gas.
 36. The process of claim 26 where the fluid is liquid.
 37. The method of claim 1 wherein the substrate and the mold are made of the same material to minimize differential thermal expansion or contraction.
 38. The method of claim 1 wherein the moldable layer includes a previously formed pattern and the mold is aligned to the previously formed pattern before pressing the molding surface and the moldable layer together.
 39. The method of claim 1 wherein the pressing by direct fluid pressure comprises filling a chamber with pressurized fluid and subjecting the mold or the substrate to pressurized fluid from the chamber.
 40. The method of claim 1 wherein the pressing by direct fluid pressure comprises: positioning a sealing device to isolate the layer of mask material from a pressure chamber; disposing the substrate and the mold in a pressure chamber; and increasing the pressure of a pressurized fluid in the pressure chamber to force together the patterned surface of the mold and the moldable layer. 